With reference to FIG. 1 and FIG. 2, the thermodynamic cycle and the schematic diagram of a conventional absorption refrigeration system are respectively shown therein. As shown in FIG. 2, the conventional absorption refrigeration system consists of a generator 31, an absorber 32, a condenser 33, an evaporator 34 and a heat exchanger 41. In such a conventional absorption refrigeration system, an aqueous solution, such as aqueous lithium bromide solution or aqueous ammonia solution, is circulating between the generator 31 and the absorber 32. During operation, cold solution flowing in pipe 1 is pumped toward the generator 31 by a pump 35 through the heat exchanger 41 in which cold solution is heated by hot solution flowing in pipe 3 and then enters the generator 31. In the generator 31, solution coming from the pipe 2 is heated again to release refrigerant vapor , and the refrigerant vapor subsequently enters the condenser 33 by way of a pipe 5. In the condenser 33, high temperature and high pressure refrigerant vapor is condensed to release heat Qc, then the condensed refrigerant flows into the evaporator 34 through a throttling device 21 which reduces the pressure of the condensed refrigerant. In the evaporator 34, refrigerant absorbs heat Qe and changes into low temperature and low pressure vapor which subsequently enters the absorber 32. In the absorber 32, low temperature and low pressure refrigerant is absorbed by solution reservoired therein and released heat Qa. Then, the above processes are repeated.
In the conventional absorption refrigeration system, the heat exchanger 41 is used for recovering part of the heat contained in the solution leaving the generator 31, so as to raise the temperature of the solution entering the generator 31. However, the specific heat and the mass flow rate of the solution flowing in the pipe 3 are less than those of the solution flowing in the pipe 1. Therefore, the temperature rise of the solution which has just absorbed heat in the heat exchanger 41 is less than the temperature drop of the solution which has just released heat in the heat exchanger 41. Furthermore, the temperature of the solution entering the generator 31 is lower than the equilibrium temperature in the generator 31. In other words, as shown in FIG. 1, solution entering the generator 31 is at state 2", however the solution reservoired in generator 31 is at state 2'. Since the pressure of the solution in the pipe 2 (at state 2a) is lower than that in the generator 31, the solution entering the generator 31 is heated first to a state whose thermodynamic pressure is equal to that in the generator 31. In the meantime, a portion of the refrigerant vapor in the generator 31 is absorbed by the solution entering thereinto. The absorbed refrigerant vapor can be used to generate a refrigeration effect if it is not absorbed by the solution entering the generator 31. Therefore, if the temperature of the solution entering the generator 31 is lower than the equilibrium temperature in the generator 31, there are three disadvantages:
1. Heat transfer occurred in the heat exchanger 41 is mainly facilitated by convection whose coefficient of heat transfer is smaller than that of the phase separation boiling heat transfer which produces vapor. Therefore, much more areas and much more tubes for heat transfer are needed, if heat transfer is facilitated by convection only.
2. Since the pressure of the solution entering the generator 31 is lower than that in the generator 31, the solution entering the generator 31 absorbs a portion of the refrigerant vapor in the generator 31 to reach the equilibrium state in the generator 31. Thus, additional heat is required to evaporate the refrigerant vapor absorbed by the incoming solution, and this additional heat occupies a very great percentage of heat Qg.
3. The pressure in the generator 31 must be raised to the design pressure to obtain a steady-state operation, thus the warm-up time is usually longer than that of a conventional vapor compression type refrigeration system. The long warm-up time is a common drawback of absorption refrigeration systems.